Getting what you need from a mask while you're on the slopes is a must. Be prepared for anything with the Magnemask Convertible Combo TNT. 4-way stretch fabrication moves freely and easily. Magnemasks are detachable faces that are held securely with magnetic seams. Ultra-thin thermax hood and neck warmer. Fleece neck warmer is contoured for added comfort. Polartec fleece, 100% polyester. To measure circumference, measure around your head, where a hat would fit comfortably.Small/Medium: 20 to 24in.Medium/Large: 22 - 26in.